For some time, it has been known to utilize hydraulic actuators connected to the linkage of an aircraft control stick to provide force to the control stick which is dependent in some fashion upon the position of the stick and other aircraft parameters, to indicate to the pilot the degree of command caused by him, which in turn is an indication of the loading of the aircraft surfaces.
In such systems, the position of the stick and other parameters are monitored with suitable transducers and a corresponding force command is generated. As the pilot moves the stick, the force changes commensurately. Such systems also generally have a trim position, which is equivalent to the old style detent wherein the force is a null at a selected position, giving the stick positional stability when in the trim position. Any change in the trim position changes the position/force relationship provided by the force command generator.
Because such force feel systems actually provide a force input to a stick, any erratic behavior thereof will provide actual commands to the control system of the aircraft, unless such force is overcome by the pilot or any automatic flight control systems. For this reason, open-loop force commands are favored only if they are implemented in sufficiently complex systems which can monitor any faulty operation and overcome it, while at the same time not impairing desired operation.
The typical hydraulic force feel system employs hydraulic servo actuators, the mechanical output of which is effective in either the forward or the reverse direction of stick motion directly on the mechanical linkage associated with the cyclic stick. The hydraulic servo actuator is controlled by an electrohydraulic servo valve which has two outputs, one relating to each of the directions of motion of the hydraulic servo actuator, the outputs having together a differential hydraulic pressure which is a function of the magnitude and polarity of a force command signal applied to the valve, the differential hydraulic pressure output determining the force (by the ratio of the area) created by the actuator. Systems of this general type are illustrated in commonly owned U.S. Pat. Nos. 3,733,039 and 3,719,336. One problem with this type of system is that a true null position (zero force for zero electrical signal input to the servo valve) is hard to maintain for long time periods over wide variations of temperature of the hydraulic fluid utilized in the servo valve and the hydraulic servo actuator. Further, amplifier drift and other factors can result in long term drift of the null. To overcome this problem, the system described in commonly owned U.S. Pat. No. 4,078,749 includes means to sense conditions in which the differential pressure should be at a null, such as during trim release with small stick motion, measuring the differential pressure across the actuator at such times, and providing a compensation bias to the system as a function of the differential pressure, which compensation is applied until the next time that a null should occur, when the compensation can be updated. This has the obvious drawback of being intermittent in keeping up-to-date on the offsets, since it does not operate continuously. In addition, this type system does not accommodate changes in hysteresis, bias, linearity and the like which occur at other than the null position.
In many servo systems, it is possible to provide closed-loop control working around a null command. For instance, in a position servo, it is possible to command a certain position, and when that position is reached as indicated by feedback signals, the command is reduced to zero. Any variation in the command results in a command error signal for repositioning the device. In such devices, the use of proportional, integral and other gains in the servo loop is relatively straightforward. However, in hydraulic force feel actuators of the type described herein, a nulling servo loop is not generally possible because the utilization of the pressure control servo valve is preferred for variety of design reasons. In such systems, the maintenance of a given pressure by the force actuator is achievable only by maintaining a differential pressure across its inputs, which in turn requires maintaining a continuous pressure command (for the desired force) at the input of the pressure control servo valve. Therefore, nulling-type servo principles cannot be employed.
Dynamic nulling to compensate for variations in the pressure control valve has been provided in a commonly-owned, copending, U.S. patent application of Clelford and Fowler, Ser. No. 087,616, now U.S. Pat. No. 4,313,165 filed Oct. 23, 1979. Therein, the pressure difference of hydraulic fluid applied to a force feel hydraulic actuator under control of a pressure control servo valve is fed back in a direct non-nulling loop that provides partial, proportional negative feedback to the signal commanding the pressure control servo valve and is also fed back in a remote loop that provides limited, nulling integral feedback to the signal commanding the pressure control servo valve. That system provides improved hydraulic force feel actuator operation in a non-nulling servo loop by means of a specifically controlled combination of direct, partial, proportional feedback and indirect, nulling integral feedback, to overcome instability, hysteresis and drift problems, and provides a measure of compensation against castastrophic failures in the force feel actuator system.
In that prior system, compensation was also provided for static null. Static null offsets, as is known, result from offsets in the hydraulic servo valve, variable friction of the mechanical control connections, differential pressure sensor (transducer) offsets, and errors in the flight control stick balance springs. The mill detent design force must be high enough to overcome all of the static forces; if the static forces of two opposite directions are unequal (null set) the design detent force must be high enough to overcome the larger of them. Thus, the more accurately the null offset can be compensated, the lower the detent force which can be employed. To provide an electrical signal value to offset these static null errors, previous static null offset systems use a manually adjusted potentiometer to create a bias, and a spring gage to measure the symmetry of control stick forces, as the bias is adjusted to various values. When the measured control stick forces seem to be completely symmetrical about the null point, the bias is deemed to be accurate. This procedure is performed by the pilot as a preflight procedure when the aircraft is on the ground. However, due to the relatively low force levels involved, it is difficult to accurately measure force symmetry with a spring gage because the friction in the control can add or substract from the measured force. Further, the rate of motion in either direction should be equal during the many iterations required to achieve the balance. Also, the spring force is measured just at the force required to start movement of the stick out of the null position, which is critical. The use of a manual method to provide an electrical signal for use as a static null offset bias is not very accurate and thus requires a higher detent force to overcome the control mechanism static friction.